In an experiment, an arm is a treatment protocol – for example, drug A, or placebo.   In medical trials, an arm corresponds to a patient group receiving a specified therapy.  The term is also relevant for bandit algorithms for web testing, where an arm consists of a specific web treatment or offer.   Assigning a web visitor to an arm (per an algorithm) is analogous to pulling one of several arms on a slot machine (also called, in this context, a “multi-arm bandit”).